Ideally, video displays such as a liquid crystal display (“LCD”) should have the ability to render continuously varying tones of all three primary colors, for example, red, green, and blue. As such, each pixel of the display would be able to generate an infinite number of colors and intensities as linear combination of the primary colors. However, a number of factors such as display physics, display memory size, driver limitations, and so on reduce the number of available color intensities.
Conventional LCDs comprise a backlight, polarization filters, other optical filters, and a liquid crystal panel which includes liquid crystal (“LC”) cells. In a liquid crystal panel, a pixel is composed of three neighboring LC cells, one for each primary color. In an LCD, a pixel's color and intensity is determined by the voltages applied to its three neighboring LC cells. Particularly, the light transmittance of each cell is a function of the voltage applied across the cell. Finally, the backlight and color filters give the otherwise monochrome cells red, green, and blue colors. The backlight may be constructed of cold cathode fluorescent lamps (“CCFL”) or light emitting diode (“LED”) arrays with optional light piping. The LCD may also include a diffuser screen to disperse the light.
For a thin-film transistor (“TFT”) LCD panel, the voltage for each LC cell is generated by a digital to analog converter (“DAC”). The voltage is strobed onto a local capacitor via a local transistor uniquely associated with that LC cell. Each LC cell must be refreshed at least at the field or frame rate of the LCD. Typical LCDs may include 6 bit DACs, which would be able to produce a total palette of 262,164 colors. More costly units may include 8 bit DACs, which would be able to produce a total palette of 16,777,216 colors. As such, large LCDs require large numbers of DACs. Moreover, due to complexity, the size of each DAC increases as the bit capacity of the DAC increases. 7 bit DACs are almost twice as large as 6 bit DACs, and 8 bit DACs are twice as large as 7 bit DACs.
In addition to information related to color, additional bits are needed to support gamma-like corrections and to zero out the local LC cell capacitor bias over the applicable temperature range. With current technology, LCDs are controlled using a total of 64 voltage levels, although, more costly LCDs may use 256 voltage levels. Nonetheless, other techniques such as spatial or temporal dithering may be used to extend the color depth and intensity range of LCDs.
Temporal dithering involves updating pixels a number of times within each pixel period. FIG. 1 illustrates an example of temporal dithering. As shown in FIG. 1, the backlight of a panel produces a uniform intensity I0 (graph 101). Each pixel interval is divided into four sub-periods T, 2T, 3T, and 4T. Each pixel is assumed to be driven by the output of a DAC either at the Trn level or the next higher level Trn+1 with the separation being δTr. Transitions may only occur at the T, 2T, 3T, or 4T markers defining the four sub-intervals of the pixel period. The transistor applies Trn+1 to an LC cell, the higher voltage for one, two, or three subintervals and Trn for the balance (graphs 102, 104, 106, 108, and 110). The human eye typically integrates the pixel's output to three intermediate values. For example, as shown in panel 104, Trn+1 is applied for sub-period T. As a result, the effective transmittance for the LC cell is Trn+0.25 δTr and the pixel intensity is I0(Trn+0.25 δTr).
Thus, by varying transmittance during the sub-periods, three extra gray shades per color are generated which produces a de-facto increase in the display color depth. However, by only getting three extra gray shades per color, the full potential of the four extra bits used by the dithering process is not being utilized.